Insulin-like growth factors (IGF's) have been isolated from various animal species and are believed to be active growth promoting molecules that mediate the anabolic effects of such hormones as growth hormone and placental lactogen. To date, several classes of IGF's have been identified. These include insulin-like growth factor-I (somatomedin C), insulin-like growth factor-II, Somatomedin A and a mixture of peptides called "multiplication-stimulating activity." This heterologous group of peptides exhibit important growth-promoting effects in vitro (Daugha-day, 1977; Clemmons and Van Wyk, 1981) and in vivo (Van Baul-Offers and Van de Brande, 1980; Schoenle, 1982).
Insulin-like growth factor-I (IGF-I) is a 70 amino acid basic protein (Rinderknecht and Humbel, 1978; Rubin et al., 1982) which has been demonstrated to play a fundamental role in postnatal mammalian growth as a major mediator of growth hormone action (Copeland et al., 1980; Zapf et al., 1981 and Schoenle et al., 1982). As such, IGF-I is useful in the treatment and/or potentiation of various growth related conditions.
The ability to obtain large quantities of IGF-I for commercial and therapeutic purposes has been hampered by the fact that the content of IGF-I in mammalian tissues is very low and, in contrast to insulin-like growth factor-II, no cultured mammalian cell lines have as yet been identified which elaborate significant quantities of IGF-I.
Natural sources of IGF-I thus far having proven inadequate to supply research and clinical needs, individuals and groups have turned to recombinant DNA techniques for the manufacture of IGF-I. Examples of such recombinant DNA methodologies can be found in European Patent Publication numbers 155,655 (published Sep. 25, 1985) which describes production of IGF-I as a cytoplasmic fusion protein; 135,094 (published Mar. 12, 1985) which describes a process for producing IGF-I as a fusion protein in E. coli; 130,166 (published Jan. 2, 1985) which describes production of IGF-I as a cytoplasmic fusion protein in Staphylococcus aureus and production as a cytoplasmic protein containing an amino-terminal methionine in E. coli; 128,733 (published Dec. 19, 1984) which describes a process for producing IGF-I in the yeast Saccaromyces cerevisiae employing a yeast .alpha.-factor, a-factor or acid phosphatase signal sequence, and describes production of IGF-I as a cytoplasmic fusion protein; Nilsson, B. et al. (1985) which describes the cytoplasmic production of IGF-I as a Staphylococcal protein A--IGF-I fusion protein; and Buell et al. (1985) which describes intracellular production of IGF-I as a mature and fusion protein.
A process for producing IGF-I as a secreted protein in Gram-negative bacteria such as E. coli has not yet been disclosed. Such a process is desirable as current methods for production of IGF-I employing cytoplasmic production have required the use of fusion protein constructs to protect the mature IGF-I protein product from degradation by cytoplasmic proteases. (see European Patent Application publication number 135,094 supra and Buell et al. 1994). Fusion protein constructs have the disadvantage of requiring an additional production step consisting of release of mature IGF from the fusion protein. Such release is typically mediated by enzymatic or chemical means, which means can be harmful to the desired IGF-I protein and/or can require modifications in IGF-I protein structure (see e.g. European Patent Application publication number 135,094 supra). Secretion of mature IGF-I into the periplasm of such Gram-negative bacteria as E. coli would afford a compartmentalization of the IGF-I between the inner and outer cell membranes of the host cells thereby protecting the IGF-I from harmful cytoplasmic enzymes. Additionally, secretion into the periplasmic space allows for production of the IGF-I as a mature protein thereby eliminating the cumbersome and often costly release of mature protein from a fusion construct or otherwise undesirable amino terminal amino acids. Furthermore, production of IGF's in E. coli avoids the possibility of undesirable glycosylation which may occur when yeast host cells are employed.
Examples of various secretion systems described for use in E. coli include U.S. Pat. No. 4,336,336 (filed Jan. 12, 1979); European Pat. Application Publication Numbers 184,169 (published Jun. 11, 1986), 177,343 (published Apr. 9, 1986) and 121,352 (published Oct. 10, 1984); Oka, T. et al. (1985); Gray, G. L. et al. (1985); Ghrayeb, J. et al. (1984) and Silhavy, T. et al. (1983). Briefly, these systems make use of the finding that a short (15-30) amino acid sequence present at the amino (NH.sub.2 -terminus of certain bacterial proteins, which proteins are normally exported by cells to noncytoplasmic locations, are useful in similarly exporting heterologous proteins to noncytoplasmic locations. These short amino acid sequences are commonly referred to as "signal sequences" as they signal the transport of proteins from the cytoplasm to noncytoplasmic locations. In Gram-negative bacteria, such noncytoplasmic locations include the inner membrane, periplasmic space, cell wall and outer membrane. At some point just prior to or during transport of proteins out of the cytoplasm, the signal sequence is typically removed by peptide cleavage thereby leaving a mature protein at the desired noncytoplasmic location. Site-specific removal of the signal sequence, also referred to herein as accurate processing of the signal sequence, is a preferred event if the correct protein is to be delivered to the desired noncytoplasmic location.
Attempts to employ such previously described signal sequences and secretion systems for delivery of heterologous proteins into the E. coli periplasmic space have, however, not always been successful. (see Kadonaga, J. et al., 1984; Ohsue, K. et al., 1983). Such results indicate that further studies are needed to clarify the secretion mechanism before secretion of heterologous proteins in such Gram-negative bacteria as E. coli can become a predictable phenomenon. The term "heterologous protein" is herein understood to mean a protein at least a portion of which is not normally encoded within the chromosomal DNA of a given host cell. Examples of heterologous proteins include hybrid or fusion proteins comprising a bacterial portion and a eucaryotic portion, eucaryotic proteins being produced in procaryotic hosts, and the like.